The present invention relates generally to metal rolling mills and more particularly to a method of compensating for tension changes in a workpiece at the entry side of a pair of opposed roller elements (work rolls) which tension changes are occasioned by thickness variations in the workpiece as it enters between the work rolls to be reduced in thickness.
In a metal rolling mill, one of the prime objectives is to produce a product having uniform thickness or gage. In a typical rolling mill, the workpiece is supplied in the form of a coil (payoff coil) from which it is unreeled and supplied, in certain instances by way of powered puller rolls, to one or more mill stands of opposed pairs of work rolls to be reduced in thickness. After reduction, the workpiece may be wound on another coil called the takeup coil. As is understood in the discipline, workpiece thickness, or gage, is essentially a function of the roll separation force and/or the tension within the workpiece. With respect to tension, that on the entry side of the work rolls is of the greater importance.
Since the amount of material delivered from payoff coil must equal the amount of material which is placed upon the takeup coil, the volume flow over a period of time is constant. As such, since in a metal rolling mill the workpiece width and the work roll speed are each essentially constant, it is apparent that the velocity of the workpiece at the entry side of a mill stand multiplied by its thickness must be equal to the product of the velocity and the thickness of the workpiece at the exit side of the stand.
From the above discussion, if it is now assumed that the exit thickness and the velocity are set, essentially, by the rolling force and work roll speed, it is intuitively apparent and mathematically true that any change in the thickness at the entry side of the stand will inversely affect the entry side velocity. This change in velocity has, in prior art controls, resulted in a change in tension at the entry side of the work rolls which in turn resulted in a change in delivery thickness and hence an "off-gage" product.
Prior art controls have provided power to the payoff coil to adjust the output torque thereof to provide the desired level of tension and to maintain tension constant as the torque arm changes when the coil diameter changes. During planned mill accelerations and decelerations of the workpiece, torque changes are made to prevent the inertia of the payoff coil system from affecting tension.
When powered puller rolls are present, the roll diameters of these drives remain constant so it is not necessary to account for changing torque arms or changing inertia. As with the payoff coil drive, torque is adjusted for obtaining the desired level of tension and for planned mill accelerations and decelerations to prevent the fixed inertia from affecting tension. These systems do not, however, compensate for velocity changes within the workpiece, due to changes in the entry thickness, which result in the change in tension and hence gage.